This invention relates to sputtering targets of high-purity aluminum or an alloy thereof. More particularly, this invention relates to sputtering targets of high-purity aluminum or an alloy thereof whose average crystal grain diameter and/or crystalline orientation is specified from an angle different from that of the prior art so as to meet the possible future requirements of semiconductor devices.
A sputtering target is usually a disk-shaped sheet which serves as a sputtering source to form electrodes, gates, wirings, elements, insulating film, protective film or the like by sputtering on substrates of various semiconductor devices. Impingement of accelerated particles upon the surface of the target causes, by momentum exchange, the emission of the atoms constituting the target to the space and their deposition on an oppositely located substrate. Sputtering targets that have been typically used include the targets of Al and Al alloys, refractory metals and their alloys (e.g., W, Mo, Ti, Ta, Zr, Nb, and their alloys such as W--Ti), and metal silicides (e.g., MoSi.sub.x, WSi.sub.x, and NiSi.sub.x).
Important among those targets are the targets of Al and Al alloys for forming Al wirings. Thin films of Al have also been used in the reflective surfaces of compact disks and optomagnetic disks.
In recent years the wafer size has been increased from 6 in. to 8 in. in diameter and the width of circuit wirings has become finer to 0.5 .mu.m or less. Following these trends in the art, the uniformity required of thin films formed by sputtering on the wafers is becoming ever stricter so that desired characteristics of resulting fine wires can be secured. Specifically, the dispersion of film thickness distribution was formerly limited to a standard deviation (.sigma.) of 5% or less, but now it is limited even to 5% or less in terms of the three fold value (3 .sigma.) of the standard deviation. With these in view, investigations are under way on sputtering equipment and conditions, and on targets, directed to the improvement in the uniformity of film thickness. It has been shown that conventional targets obtained hitherto are of such quality that the dispersion and variation of film thickness distribution are still large from target to target and during services of the same target, and they apparently fail to satisfy the afore-said standard (3 .sigma.&lt;5% ).
In connection with the dispersion of film thickness distribution which is attributable to the quality of the Al or Al alloy target used, it is presumed that, because the surface and internal crystal structure (i.e., the crystal grain diameter, crystalline orientation, etc.) materially influences the emission of atoms from the target, the non-uniformity of crystal structure of the target has a significant effect upon the thickness distribution of the thin film formed by sputtering on a wafer. In view of this, it has been attempted with high-purity aluminum or alloy target to make it as fine in crystal structure as possible and control certain crystal planes for crystalline orientations.
For example, a paper "Crystallographic target effects in magnetron sputtering" by C. E. Wickersharm, Jr. which was carried in J. Vac. Sci. Technol. A. Vol. 5, No. 4, Jul/Aug 1987, pp. 1755-1768, reports on the influence of crystalline orientation on the uniformity of sputtered thin films. Japanese Patent Application Public Disclosure No. 312975/1988 describes, in view of the fact that a thin film of aluminum formed by sputtering on a wafer shows such a thickness distribution is thick in the center and thin along the periphery, an aluminum sputtering target in which the crystalline orientation content ratio {220}/{200} is larger in the center than in the periphery. Japanese Patent Application Public Disclosure No. 15167/1990 describes an aluminum sputtering target having the (111) crystal plane accounting for at least 50% of the surface area. Patent Application Public Disclosure No. 2369/1991 proposes decreasing the crystalline orientation intensity ratio {100}/{110} gradually from the target surface toward the innermost region so as to overcome the problem of magnetron sputtering, i.e., the deterioration of film thickness distribution due to a change in the direction of atom emission concurrently with the formation of a ring-shaped groove on the surface along the path of rotation of the magnet with the consumption of the aluminum target. Patent Application Public Disclosure No. 10709/1991 discloses an aluminum target characterized in that the surface to be sputtered has a crystalline orientation content ratio {220}/{200} of 0.5 or more. Further, Patent Application Publication No. 246170/1992 teaches an aluminum target having a grain size of 2 mm or less and a &lt;110&gt; fiber structure with a fiber axis having an X-ray diffraction intensity of at least 20 times the random value.
Still, these targets are unable to satisfy the specification that the threefold value (3.sigma.) of the standard deviation be 5% or less.